What form do i use if i am filing married filing single?

There is only one instane that you should file married filing separately, it is by far the worst way to file, if you are married use the standard deduction if you do not itemize, the only way I would file separately (even if I am separated from my wife) is if she won a million dollars in the lottery and refused to report it on our tax report, I do not want to go to prison with her so I would file separate and have nothing to do with it. You would use a normal 1040 or 1040EZ/ A and at the top check "married filing separately". You gain the same standard deduction as filing single and you can only claim children once per return (another reason to file jointly). Whoever files taxes first will gain the benefit of claiming the children.